straight up vs LKO+Oberth effect

[lIAceI]

sorry this has been debated to death, but i can't get a definitive answer.

so, everywhere i read LKO+Oberth is the best way to get out of kerbin's SoI.

the reasoning behind this is while going straight up the rocket has to fight gravity for the entire trip.

however, your doing the exact same thing when gaining orbit, just in a different direction, to get a "constant fall", and increasing apoapsis is fighting gravity so you fall back to the planet.

i decided to do some testing to see which method will have the greatest effect on the sun's periapsis and deltaV at escape.

same craft in each test

relevant mods: NEAR and kerbal engineer redux(for the data)

1+TWR

6405 total DV

400SI engine

a few small wings to keep foreword

here are the results

target was lowest sun periapsis.

LKO to escape

DV at escape 1881

sun periapsis 5 300 042 585 (after all fuel was used)

note: i had to burn several times in lko, all at periapsis.

straight up

DV at escape 1671

sun periapsis 3 041 693 328 383 (after all fuel was used)

i am by no means a expert, or even an enthusiast space engineer, but both results are fairly similar. with the straight up approach sacrificing some deltaV into lower periapsis.

the straight up method has it's drawbacks, needing 1+TWR for the entire burn and you cannot rendezvouses with other crafts.

the LKO+Oberth method is interesting compared to straight up, i had more periapsis, and more fuel after leaving kerbins SoI.

LKO+Oberth is much more complicated to get out of kerbin's SoI, needing several precise burns at just the right time to be effective. then there's the difficulty of getting orbit. many steps that need to be done with little room for error (which is bad in any situation).

i prefer the straight up method for it's simplicity. since im burning at the suns AP or PE from the start.

just for kicks, i decided to do LKO+straight up, gaining 1 000 000 altitude, then starting my orbit maneuver followed by a escape maneuver.

sun periapsis 7 748 530 240

DV at escape 1481

these results are probably skewed by my inexperience, as i got frustrated by LKO+Oberth very early on and started shooting straight up instead.

can anyone else post results?

this is the only one i could find, from reddit:

voneiden

Actual research results:

3 stage craft. Each stage burns for 70 seconds.

1) Standard launch. At 10km, tilt 45 degrees east. Circularize at ~100-125km. Burn everything left and coast to escape. v∞ = 5037m/s.

2) Launch straight up. UP UP UP. Coast to escape. v∞ = 5112m/s.

Conclusions

Delta-v was probably lost in case 1 due to slight burning to assist in vessel rotation. As such, I conclude both methods to be equal in their efficiency. Method 2 suffers from burning at high altitude, while method 1 suffers from the extra cost of circularizing (but gains boost from lower altitude final burn). Low thrust vehicles should stick to method 1 so they can benefit from oberth effect. High thrust vehicles might wanna stick with method 2.

[The_Rocketeer]

This really has been discussed to death.

The reason why a gravity turn is better (and it is) is this:

When you launch to escape in a straight line, you accelerate at "thrust - gravity" all the way to escape. Very simple.

BUT...

When you launch and prograde gravity turn to escape, you fly in 3 distinct phases:

Phase one: acceleration at thrust - gravity.

This lasts until your gravity turn gets going and your trajectory becomes 'past Kerbin' rather than 'away from Kerbin'.

Phase two: acceleration at thrust + gravity.

This starts when your trajectory becomes 'past Kerbin' rather than 'away from Kerbin' and lasts as long as it takes for your trajectory to go back to being 'away from' rather than 'past'. During this time, you do most of the accelerating you'll need. Remember gravity is pulling you in the direction of Kerbin's centre, but relative to your rocket this is now much nearer 'sideways' than 'backwards', so it isn't really reducing your forward acceleration anymore, but it IS increasing your sideways acceleration. In other words, your total acceleration is now a synergy, rather than a subtraction.

Phase three: acceleration at thrust - gravity again.

By now, you're going so fast that you probably don't have that much accelerating left to do. Because you're going faster it takes less time to get to escape altitude, so gravity has less time to work on your ship.

I've tried to explain this quite figuratively, which works for me. If someone who's got some equations to show this can illustrate it that way I'd appreciate the back-up

[Red Iron Crown]

Straight up from Kerbin's surface isn't practical for escaping Kerbin if you're going to another planet. Your exit vector really has to be pretty precise to actually get somewhere useful; that requires careful timing and KSP's maneuver nodes are no help for a launch-straight-to-escape profile. I think any dV savings from launching direct to escape are going to be offset by larger corrections on an interplanetary mission.

[The_Rocketeer]

Straight up from Kerbin's surface isn't practical for escaping Kerbin if you're going to another planet. Your exit vector really has to be pretty precise to actually get somewhere useful; that requires careful timing and KSP's maneuver nodes are no help for a launch-straight-to-escape profile. I think any dV savings from launching direct to escape are going to be offset by larger corrections on an interplanetary mission.

This is my point.

There are no dV savings from this approach.

[Red Iron Crown]

This is my point.

There are no dV savings from this approach.

There are, you never raise your periapsis from deep below Kerbin's surface in a direct-to-escape launch, which you must do if you circularize first.

[lIAceI]

can you test it, to see just how significant the Dv savings are?

i gave up on LKO and orbit awhile ago, if i need more fuel i just send a fuel drone the next day. this way i only need to worry about fuel capacity and TWR.

completely inefficient, but i don't need to screw around in kerbin.

[Signo]

Guys, I can see the d/v advantages of a direct trajectory, but the overall set up and focus required make me a fan of the LKO solution.

I mean, we are not flying torchships that need to follow a "brachistocrona" (I do not know how to write it in english and I was lazy to Google it, I hope it is almost the same).

X-wing anybody?

[numerobis]

This thread should perhaps be merged with this one?

The conclusion is that it is significantly more expensive to go straight up than to go into orbit at 70km via a proper gravity turn, and then do your transfer burn.

How much depends on your TWR -- if you have ridiculous thrust, the two approaches end up looking about the same. However, if you have ridiculous thrust, you can go to the VAB and remove some engines, and thereby end up with more deltaV on your spacecraft.

[Kryxal]

The third thing to try is executing a gravity turn, then just don't try to keep your apoapsis close and don't worry about raising periapsis ... I have a feeling this will be a touch more efficient than going to LKO first.

[GoSlash27]

I posted results on a test of this in the other thread:

My DV figures from my test using the Pathfinder:

1) Prograde gravity turn with direct injection: 5230 m/sec

2) Prograde orbit followed by injection: 5,260 m/sec (+30 m/sec)

3) Vertical direct injection: 5,570 m/sec (+340 m/sec)

These were launches to hit Munar apoapsis to see which approach took the least DV.

The profiles were as follows:

TWR for all cases was a maximum of 2G. I held the TWR to the standard for typical prograde profiles* and left it running hot for the vertical ascent at 2G for a reverse terminal velocity fall into space for the vertical.

* Prograde profile was gravity kick at 7KM, 68* pitch at 15KM, 45* pitch at 25KM, and 23* pitch at 35KM with a constant, linear reduction throughout. 1 -> 2G acceleration in the vertical boost phase, .75G acceleration in the transstage phase, and .5G in the injection phase.

TL/DR:

Going vertical is never going to be more efficient regardless of where you're going. If you just want to do it because it's easier, then have at it. Just remember that you will need padding in your budget. The reduced Oberth effect from a vertical launch means that this method will require more and more budget padding the further away your destination.

Best,

-Slashy

[MarvinKitFox]

There are, you never raise your periapsis from deep below Kerbin's surface in a direct-to-escape launch, which you must do if you circularize first.

yes.

And the amount of delta-v you save is exactly equal to how much delta-v it would take to raise your periapsis **when at apoapsis**

If you are going to Mun, this is about 38m/s

If you are going to Minmus, this is about 7m/s

For the OP's scenario, it is well under 10^-6 m/s

[arkie87]

The reduced Oberth effect from a vertical launch means that this method will require more and more budget padding the further away your destination.

Best,

-Slashy

The Oberth effect is utilized in both cases since you are doing the majority of the burning pro-grade, but i suppose what you mean is that gravity will continue suck down your velocity when going straight vertical, and therefore, you will have to use more fuel to get up to the same speed to use Oberth

[Srpadget]

I'm always a little confused by these sorts of debates, which basically boil down to:

"Which technique saves me the most dV? The much more complicated one that has been used in real-life by every space program since the 1950's and is the end result of tens of thousands of very smart, highly-educated specialists...who all came up with the exact same answer independently? Or this way simpler technique which I thought up on my own?" (Bonus points when none of the discussion uses math.)

I don't think that's what they're TRYING to say, really I don't. But that's pretty much what it boils down to.

My take on it? If it seems obvious, commonsense, and easy, but it's not done that way in real life--there's probably a reason for that. (Exceptions for anything involving KSP's grossly overpowered jet engines and/or where the physically-wrong stock drag model is involved.)

[GoSlash27]

The Oberth effect is utilized in both cases since you are doing the majority of the burning pro-grade, but i suppose what you mean is that gravity will continue suck down your velocity when going straight vertical, and therefore, you will have to use more fuel to get up to the same speed to use Oberth

Well, what I mean is that bit in the middle: "gravity will continue suck down your velocity when going straight vertical". There is no speed at which Oberth suddenly comes into play. The fact that you expended all that fuel at a lower velocity the entire time means that you didn't get as much kinetic energy out of it as you otherwise would have.

The fact that you had less velocity at the end of it all also means that any subsequent burns would not give you as much kinetic energy as they could have.

Best,

-Slashy